1. Field of the Invention
The present invention relates to a golf ball excellent in durability while having high hardness and high stiffness, and more specifically, relates to an improvement in a resin composition to be used for an outer layer of a golf ball.
2. Description of the Related Art
As base resins used in an outer layer for covering a core of a golf ball, such as a cover layer or an intermediate layer, ionomer resin or polyurethane are used. A thread-wound core or a vulcanized rubber-molded core is wrapped with a resin composition including these resins as main components.
In the above structure, in order to achieve a long ball distance, a cover layer is preferably formed as an outer layer having high hardness and high stiffness to provide high resilience. However, such a cover layer having high hardness and high stiffness has a problem that it tends to get cracked after hit repeatedly and its durability deteriorates.
In an attempt to solve the problems described above, it has been proposed to include fibrous aluminum borate whiskers or organic short fibers (Japanese Laid-Open Patent Publication No. 10-137365). However, these fibrous substances have small specific surface areas. Because of such structures, although the fibrous substances are easily dispersed, large reinforcing materials come to scatter over the resin. When dispersed in the matrix of the resin, the reinforcing materials have small reinforcing effect. Therefore, in order to satisfy a desired mechanical property, it is required to blend a large amount of reinforcing materials. As a result, the resin content in the resin composition lowers. In this case, neither the resilience which is important as a golf ball nor the durability at a desired level is achieved.
Further, it has also been proposed to add nanocomposite material such as hydrotalcite or octosilicate as a reinforcing material other than those described above into a resin composition (Japanese National Publication No. 2004-504900). Since the fillers of these types are formed into nano-sized fine particles, they are hard to be dispersed into resins. In particular, such fillers are difficult to be dispersed as particles into hydrophobic resins such as ionomer resin and polyurethane. As a result, agglomerated particles are unevenly located in the resin matrix, as is the case of the reinforcing materials with large diameters. Therefore, even if these reinforcing materials are used, neither the hardness of desired level nor the sufficient durability is obtained.